The present invention relates to communications methods and apparatus, and more particularly, to spread spectrum communications methods and apparatus.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, FTACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modem digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use so-called xe2x80x9cdirect sequencexe2x80x9d spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
Typical transmit operations of such a system are illustrated in FIG. 3. Data streams from different users are subjected to various signal processing steps, such as error correction coding or interleaving, and spread using a combination of a user specific spreading code and a group-specific scrambling code. The coded data streams from the users are then combined, subjected to carrier modulation and transmitted as a composite signal in a communications medium.
A so-called RAKE receiver structure is commonly used to recover information corresponding to one of the user data streams. In a typical RAKE receiver, a received composite signal is typically correlated with a particular spreading sequence assigned to the receiver to produce a plurality of time-offset correlations, a respective one of which corresponds to an echo of a transmitted spread spectrum signal. The correlations are then combined in a weighted fashion, i.e., respective correlations are multiplied by respective weighting factors and then summed to produce a decision statistic. The performance of CDMA systems generally is limited by interference among different user signals. Spreading/despreading provides a degree of interference suppression, but the number of users is generally limited by interference.
Conventional RAKE reception techniques generally treat interference as white noise. More recently proposed techniques provide for a degree of interference cancellation through xe2x80x9cwhiteningxe2x80x9d of interference. Examples of such techniques are described in xe2x80x9cA Noise Whitening Approach to Multiple Access Noise Rejection-Part I: Theory and Background,xe2x80x9d by Monk et al., IEEE Journal on Selected Areas in Communications, vol. 12, pp., 817-827(June 1994); xe2x80x9cA Noise Whitening Approach to Multiple Access Noise Rejection-Part II: Implementation Issues,xe2x80x9d by Monk et al., IEEE Journal on Selected Areas in Communications, vol. 14, pp. 1488-1499 (October 1996); xe2x80x9cData Detection Algorithms Specifically Designed for the Downlink of CDMA Mobile Radio Systems,xe2x80x9d by Klein, 1997 IEEE Vehicular Technology Conference, Phoenix Ariz. (May 4-7, 1997); U.S. Pat. No. 5,572,552 to Dent et al. (issued Nov. 5, 1996); and xe2x80x9cOptimizing the Rake Receiver for Demodulation of Downlink CDMA Signals,xe2x80x9d by Bottomley, Proceedings of the 43rd IEEE Vehicular Technology Conference, Secaucus N.J. (May 18-20, 1993).
Unfortunately, these approaches can be highly complex and difficult to implement in a practical receiver. Whitening approaches tend to provide dramatic gains when the number of RAKE fingers greatly exceeds the number of resolvable multipaths in a received signal, and thus a complex receiver design may be needed in order to obtain performance gains offered by such approaches. This complexity can be amplified in soft-handoff situations, in which a terminal is simultaneously receiving signals from multiple base stations. In addition, wide bandwidth next-generation CDMA systems, such as wideband CDMA (WCDMA) and cdma2000, may require even more complex receiver designs due to increased numbers of multipaths.
In light of the foregoing, it is an object of the present invention to provide improved methods and apparatus for recovering information represented by a spread spectrum signal transmitted in a communications medium.
It is another object of the present invention to provide improved methods and apparatus for recovering information represented by a spread spectrum signal that can compensate for interference from other spread spectrum signals transmitted in the communications medium.
It is another object of the present invention to provide improved methods and apparatus for recovering information from spread spectrum signals that can be implemented using less complex receiver operations and architectures than conventional interference suppression techniques.
These and other objects, features and advantages can be provided, according to the present invention, by methods and apparatus in which a composite signal including a spread spectrum signal is correlated with a desired spreading sequence, and the resulting correlations are subjected to a multistage combining process in which respective groups of correlations are combined to produced intermediate combined values that are subsequently combined in a manner that compensates for correlated impairment in the composite signal. The first stage of combining preferably is a traditional RAKE combining process in which correlations are combined according to channel estimates. The second stage of combining may use a number of different techniques for canceling multiuser interference and other correlated impairment, including explicit computational techniques, interference rejection combining (IRC), or adaptive techniques. Using a multistage approach allows the number of finger elements combined in more complex interference-canceling combining operations to be reduced, thus offering a potential reduction in complexity. The multistage approach also lends itself to hybrid combining techniques.
In particular, according to aspects of the present invention, information encoded in a spread spectrum signal transmitted in a communications medium is recovered. A composite signal including a spread spectrum signal is received from the communications medium, and correlated with a spreading sequence to generate time-offset correlations. Respective first and second groups of the correlations are combined, e.g., according to estimated channel coefficients, to produce respective first and second combined values. The first and second combined values are then combined in a manner that compensates for correlated impairment in the composite signal to generate an estimate information in the transmitted spread spectrum signal.
According to an embodiment of the present invention, the first and second combined values are combined based on an estimated impairment correlation and on a composite channel response that reflects the preceding combining operations. The composite channel response and impairment correlation may be estimated and used to generate weighting factors that are used to combine the first and second combined values. The weighting factors may also be iteratively generated from an estimated composite channel response, an estimated impairment correlation, and previously determined weighting factors. According to another embodiment of the present invention, the first and second combined values are combined according to weighting factors that are adaptively estimated based on a comparison of combined values generated by the second combining stage to a reference value, such as a pilot symbol value or a decoded symbol value.
According to another aspect of the present invention, an apparatus for recovering information encoded in a spread spectrum signal included in a composite signal includes a correlation unit operative to correlate the composite signal with a spreading sequence to generate time-offset correlations. A multistage combiner is responsive to the correlation unit and operative to combine respective groups of the correlations to provide respective intermediate combined values, and to combine the intermediate combined values in a manner that compensates for correlated impairment in the composite signal to estimate information in the transmitted spread spectrum signal.
According to one embodiment of the present invention, the multistage combiner includes a first combiner responsive to the correlation unit and operative to combine a first group of the correlations to produce a first combined value. A second combiner is responsive to the correlation unit and operative to combine a second group of the correlations to produce a second combined value. A third combiner is responsive to the first combiner and to the second combiner and operative to combine the first and second combined values in a manner that compensates for correlated impairment in the composite signal to generate an estimate of information in the transmitted spread spectrum signal.
The first and second combiners may combine the first and second groups of correlations in a manner that compensates for effect of a channel over which the spread spectrum signal is received, e.g., using channel coefficient estimates. The third combiner may be operative to combine the first and second values based on an estimated impairment correlation and on a composite channel response that reflects effects of the first and second combiners, e.g., using weighting factors determined from an estimated composite channel response and an estimated impairment correlation. Alternatively, the third combiner may combine the first and second values according to adaptively estimated weighting factors.